1. Field of the Invention
This invention relates to a ridge-waveguide semiconductor laser.
2. Description of Related Art
FIG. 2 is a sectional view illustrating the basic construction of a ridge-waveguide semiconductor laser according to the prior art.
As shown in FIG. 2, the conventional ridge-waveguide semiconductor laser includes an n-type GaAs substrate 1, an n-type AlGaAs first cladding layer 2, a GRIN-SCH (Graded-Index Waveguide Separate Confinement Heterostructure) AlGaAs active layer 3, a p-type AlGaAs second cladding layer 4, a p-type AlGaAs third cladding layer 5, and a p-type GaAs cap layer 6.
The construction of this ridge-waveguide semiconductor laser is such that the third cladding layer 5 and cap layer 6 project from the second cladding layer 4, with this protrusion being formed by etching. The etching process entails removing the third cladding layer 5 employing a selective etching solution usually exhibiting selectivity the boundary of which is x=0.5 in terms of the Al composition ratio in Al.sub.x Ga.sub.1-x As, and therefore the Al Composition ratio of the second cladding layer 4 is made x=0.4 and the Al composition ratio of the third cladding layer 5 is made x=0.6.
Accordingly, in the case of the conventional ridge. waveguide semiconductor laser, the Al composition ratio of the second cladding layer 4 cannot be made very high (i.e., no higher than x=0.5) on account of the selective etching solution employed. As a result, a short-wavelength lasing-type semiconductor laser cannot be obtained.
More specifically, it is necessary to raise the composition ratio of Al in the active layer 3 in order to shorten the oscillation wavelength of a semiconductor laser. To satisfy the basic requirement that the Al composition ratio in the active layer 3 be smaller than the Al composition ratio in the second cladding layer 4, obviously the Al composition ratio in the second cladding layer 4 must be enlarged.